This invention pertains to improvements in power tools for tightening threaded fasteners wherein mechanisms are provided for driving the output spindle of the tool at high speed during the relatively free running portion of the tool operating cycle before the fastener strongly resists rotation, and then driving the output spindle at a relatively low speed during the final tightening process in order to produce a desired final torque on the fastener. Such mechanisms are usually provided in fastener torquing tools instead of providing the tool with a relatively large motor the capacity of which is not needed during the free running portion of the tool operating cycle. Known devices in the field of the present invention include apparatus such as that disclosed in U.S. Pat. No. 3,430,521 to M. L. Kulman and U.S. Pat. No. 3,610,343 to S. A. Bratt.
U.S. Pat. Nos. 3,739,659 and 3,960,035 assigned to the assignee of the present invention represent further improvements in speed shifting mechanisms for power tools. The inventions disclosed in the two last mentioned patents include pressure fluid actuators for holding a torque responsive clutch disengaged to effect speed shifting. Such mechanisms are particularly advantageous for use in pneumatic multiple tools arrangements wherein it may be desired to effect the shifting of all tools simultaneously. However, for single tool installations the pressure fluid actuated shifting mechanisms is usually more expensive and requires careful maintenance, and, of course, pressure fluid actuated speed shift mechanisms cannot be used conveniently with electric motor driven tools or the like where a source of pressure fluid is normally not available.